Chapter 2.
The Chemical Context of Life
AP Biology
2005-2006
Why are we studying chemistry?
 Biology has chemistry at its foundation
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2005-2006
The Basics
 Everything is made of matter
 Matter is made of atoms
 Atoms are made of:
protons
 neutrons
 electrons

+
0
-
mass of 1
mass of 1
mass <<1
nucleus
nucleus
orbits
 Different kinds of atoms = elements
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2005-2006
The World of Elements
ology
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2005-2006
Models of atoms
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(c)
2005-2006
Atomic structure determines behavior
 The number of protons in an atom
determines the element
# of protons = atomic number
 this also tells you # of electrons

 All atoms of an element have same
chemical properties
all behave the same
 properties don’t change

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2005-2006
Life requires ~25 chemical elements
 About 25 elements are essential for life

Four elements make up 96% of living
matter:
• carbon (C)
• oxygen (O)

Four elements make up most of
remaining 4%:
• phosphorus (P)
• sulfur (S)
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• hydrogen (H)
• nitrogen (N)
• calcium (Ca)
• potassium (K)
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5-2006
Isotopes
 Different number of neutrons (heavier)
 Some are unstable

nuclear reactions / decay
 Split off neutrons &/or protons

radioactivity
 Biological tool
 Biological hazard
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2005-2006
Bonding properties
 Effect of electrons
chemical behavior of
an atom depends on its
electron arrangement
 depends on the
number of electrons in
its outermost shell, the
valence shell

How does this
atom behave?
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2005-2006
Bonding properties
 Effect of electrons

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chemical behavior of an atom depends on
number of electrons in its outermost shell
How does this atom behave?
How does this atom2005-2006
behave?
Elements & their valence shells
 Elements in the same row have
the same number of shells
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2005-2006
Elements & their valence shells
 Elements in the same column
have the same valence & similar
chemical properties
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2005-2006
Elements & their valence shells
 Moving from left to right, each
element has a sequential addition
of electrons (and protons)
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2005-2006
Chemical reactivity
 Atoms tend to
Complete a partially filled outer
(valence) electron shell
or
 Empty a partially filled outer (valence)
electron shell


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This tendency drives chemical reactions
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Ionic bonds
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2005-2006
“Let’s go to the video tape!”
(play movie here)
Ionic bonds
 Transfer of an electron
 Forms + & - ions
+ = cation
 – = anion

 Weak bond
 example:

salt = dissolves
easily in water
20
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05-2006
Covalent bonds
 Two atoms need an electron
 Share a pair of electrons
 Strong bond

both atoms holding onto the electrons
 Forms molecules
 example:

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water = takes energy to separate
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Double covalent bonds
 Two atoms can share more than one
pair of electrons
double bonds (2 pairs of electrons)
 triple bonds (3 pairs of electrons)

 Very strong bonds
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Multiple covalent bonds
 1 atom can form covalent bonds with
two or more other atoms
forms larger molecules
 ex. carbon

Biology
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2005-2006
Polar covalent bonds
 Pair of electrons not shared equally by

2 atoms
Water = O + H
oxygen has stronger
“attraction” for the
shared electrons than
hydrogen
 oxygen has higher
electronegativity

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2005-2006
Polar covalent bonds
 2 hydrogens in the water molecule

form an angle
Water molecule is polar
oxygen end is negative
 hydrogen end is positive

 Leads to many
interesting properties
of water….
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“Let’s
go to the video tape!”
(play movie here)
2005-2006
Hydrogen bonds
 Positive H atom in


1 water molecule is
attracted to negative O
in another
Can occur wherever an
-OH exists in a larger
molecule
Weak bonds
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2005-2006